1. Technical Field of the Invention
The present invention relates to stators for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of the Related Art
In recent years, electric rotating machines, such as electric motors and electric generators, have been required to be compact, be able to output high power, and have high quality. In particular, for electric rotating machines for use in motor vehicles, the spaces available for installation of those machines in the motor vehicles have been decreasing, while the need for them to output high power has been increasing.
Generally, for making an electric rotating machine compact while enabling it to output high power, it is required to: reduce the resistances of rotor and stator coils of the machine; improve the space factors of electric wires, which form the stator coil, in slots of a stator core of the machine; and arrange regularly and densely those portions of the electric wires which protrude outside of the slots of the stator core.
To meet the above requirements, PCT International Publication No. WO 2008/108351 A1 and Japanese Patent First Publication No. 2000-139048, both of which have the same assignee as the present application, disclose techniques for improving stators for electric rotating machines. Those stators each include a hollow cylindrical stator core and a stator coil mounted on the stator core. The stator core has a plurality of slots that are formed in the radially inner surface of the stator core and spaced in the circumferential direction of the stator core at predetermined intervals. The stator coil is made up of a plurality of electric wires wave-wound around the stator core. Specifically, each of the electric wires includes a plurality of in-slot portions, which are received in the slots of the stator core, and a plurality of turn portions that are located outside of the slots to connect the in-slot portions.
According to one technique disclosed in PCT International Publication No. WO 2008/108351 A1, each of the turn portions of the electric wires is configured to include a plurality of shoulder parts that extend parallel to an axial end face of the stator core. With the shoulder parts, it is possible to densely arrange the turn portions of the electric wires and to reduce the protruding height of the turn portions from the axial end face of the stator core.
According to another technique disclosed in Japanese Patent First Publication No. 2000-139048, the stator coil is formed by (1) interlacing the electric wires in a predetermined manner to form a flat band-shaped electric wire assembly; and (2) rolling the flat band-shaped electric wire assembly by a predetermined number of turns to form a hollow cylindrical electric wire assembly which makes up the stator coil. With this technique, it is possible to reduce the radial width of the turn portions of the electric wires. In addition, the stator coil disclosed in PCT International Publication No. WO 2008/108351 A1 is also formed using the above technique.
However, with the above technique for forming the stator coil, when the number of the electric wires is large, the process of interlacing the electric wires may become very complicated, thus increasing the manufacturing cost of the stator coil.
Therefore, the inventors of the present invention have investigated a simpler method of forming the flat band-shaped electric wire assembly. As a result, they have concluded that the manufacturing cost of the stator coil may be reduced by forming the flat band-shaped electric wire assembly by simply stacking, instead of interlacing, the electric wires.
However, in this case, if the electric wires were not properly shaped, the thickness of the resultant flat band-shaped electric wire assembly would vary cyclically in the longitudinal direction of the assembly. Moreover, the variation in the thickness of the assembly would be greater than that in the case of forming the assembly by interlacing the electric wires. Consequently, in the stator coil that is obtained by rolling the flat band-shaped electric wire assembly into the hollow cylindrical shape, there would be formed protruding portions which protrude radially inward from the radially inner surface of the stator coil or radially outward from the radially outer surface of the stator coil. As a result, after assembling the stator coil to the stator core, as illustrated in FIG. 20, some of the in-slot portions of the electric wires would protrude radially inward from the corresponding slots of the stator core, forming void spaces at the bottoms of the corresponding slots. With the void spaces, the space factors of the electric wires in the corresponding slots would be decreased, thereby lowering the performance of the stator. Here, the space factor for each of the slots of the stator core is defined as the ratio of the sum of cross-sectional areas of those in-slot portions of the electric wires which are actually received in the slot to the cross-sectional area of the slot.